The invention relates to an electrically conductive wire having a solder ball thereon for making a circuit connection between electrically conductive members on opposite surfaces of a substrate by means of a soldering process.
For circuit connection across both surfaces of a substrate, a through-hole substrate is known from EP-A-0 126 164. For making the circuit connection an electrically conductive wire having a solder ball thereon is used. The wire is inserted in the through-hole of the substrate. The substrate has electrically conductive members on both surfaces and the conductive members are arranged on the perimeters of the through-hole. The solder ball on the wire is positioned on one of the conductive members. By means of a soldering process, like dipping or flow soldering, one or both ends of the wire are soldered to a conductive member at one surface of the substrate, whereas at the same time heat generated during the soldering process is transferred through the wire to the solder ball at the opposite surface of the substrate in order to melt the solder ball for making a soldering connection between the wire and the conductive member at the opposite surface of the substrate. In this way the circuit connection between the conductive members on both surfaces of the substrate is made.
In practice it has turned out that to make a circuit connection with the use of a single solder ball on it as described hereinbefore, often leads to a bad connection between the conductive members.
Therefore many manufacturers use a separate soldering robot for making such circuit connections. However, this method is rather expensive and results in a limitation in design, logistics, maintenance and process control.
The object of the invention is to provide an electrically conductive wire means for making a reliable circuit connection between electrically conductive members on opposite surfaces of a substrate.
The present invention provides an electrically conductive wire having a solder ball thereon for making a circuit connection between electrically conductive members on opposite surfaces of a substrate, characterized in that the wire comprises two spaced apart solder balls on it and flux in the space between the solder balls. Using this kind of wires for making circuit connections across both surfaces of a substrate results in a good and reliable connection. The wires are very suitable for use in an automatic soldering process, like reflow or wave soldering. The wire can be inserted in the through-hole of a substrate during the process of placing other components with the same machine. In one of the subsequent operation steps the components are soldered at their required position. At the same time the wires can be soldered in the same soldering process. So, the most important advantage is that the wires can be soldered during existing automatic soldering processes of other components. Preferably the solder balls are composed of tin, lead and bismuth instead of tin and lead only as usually used. the addition of bismuth reduces the melting temperature of the solder.